Final answer:
The question is related to photography and the use of chemicals in film development. While a common guideline is to change developer and fixer solutions every 3 to 4 weeks, the exact time can vary based on use and specific chemical types.
Step-by-step explanation:
The question about how often developer and fixer should be changed is related to photography, which falls under the subject of Arts. These chemicals are used in developing analog film in a darkroom. While there is no hard and fast rule for every situation, a common guideline for amateur and hobbyist photographers is to change developer and fixer solutions every 3 to 4 weeks to ensure quality, regardless of use. However, this can vary significantly depending on the frequency of use, the volume of development being done, and the specific type of chemicals. Professional environments or high-volume darkrooms might need to change their chemicals more frequently to maintain consistency and quality in their photographic outputs.
Assuming the chemistry does not specify a shelf life, and there is no noticeable degradation in performance, sticking within a few weeks range and monitoring results should provide adequate guidance for when to switch out the solutions.